Organic electronic devices have drawn a great deal of research interest in recent years because of their potential for broad commercial application, including electroluminescence devices, field effect transistors and organic solar cells, etc. In all these devices, the key component is organic semiconducting materials, which are usually used as active thin layers. To get satisfactory device properties and performance, the chemical structures of these organic materials must be carefully controlled and optimized.
Among organic semiconductors, alternating conjugated polymers of an electron donor (ED) unit and an electron acceptor (EA) unit have attracted more and more attention due to their special properties associated with the donor/acceptor (D/A) structure in the main chain. This D/A structure can effectively lower the band gap of conjugated polymers, which is very important, especially for solar cell applications, where the polymer absorption should be fine-tuned to match the solar spectrum. Meanwhile, the energy offset between lowest unoccupied molecular orbital (LUMO) of the polymer and the fullerene derivatives (widely used electron acceptors in organic solar cells) should be well controlled to be just enough for charge separation in order to minimize energy loss. However, to fine tune the energy levels (HOMO, LUMO) of the conjugated polymer, and at the same time, optimize other properties, such as solid state packing, solubility, carrier mobility still tends to be difficult.
Fluorinated conjugated polymers show several advantages compared with non-fluorinated counterpart. First, they usually have lower HOMO and LUMO energy levels, which will increase open circuit voltage of photo voltaic devices and endow the polymer better resistance against the oxidation degradation process. Second, because of high electronegativity of fluorine, the resulting polymers can be used as n-type or ambipolar semiconducting materials. Third, sometimes, they can form C—H . . . F interactions, which can influence the solid state supramolecular organization, phase segregation and π-π stacking. This may enhance the charge carrier mobility. However, the number of fluorinated monomers with strong electron withdrawing ability is quite limited.
It is known that a monomer as illustrated in Scheme 1 is a strong electron acceptor unit exhibiting good properties in optoelectronic device applications (Zhang 2004).
However, there are only a very limited number of methods to successfully introduce fluorine atoms on to an organic molecule. Two major methods have been reported to introduce fluorine atoms into an aromatic ring. The first, and most widely used method, uses the Balz-Schiemann Reaction. This approach involves conversion of aryl amines to aryl fluorides via diazotisation and subsequent thermal decomposition of the derived tetrafluoroborates or hexafluorophosphates. The second method uses butyl lithium and a special fluorinating agent, such as N-fluorobenzenesulfonimide. These two methods are usually tedious and involve multi-step synthesis. Very stringent reaction conditions are also usually involved which may not be compatible with many organic groups, especially with some groups having strong electron withdrawing properties, such as 2,1,3-benzothiadiazole. For these reasons, monomers containing fluorine and at the same time having strong electron withdrawing properties are quite limited in the art. One report describes fluorinated monomers and polymers containing 3-substituted-4-fluorothiophene units (Heeney 2004).
There remains a need for new monomers having improved electronic properties for use in producing new polymers for use in electronic devices.